Particle separation in microfluidics using a switching ultrasonic field.

We present a new method for separation of micro-sized constituents with positive acoustic contrast factors in a microfluidic channel using ultrasound. The ultrasound field is switched between the first and third resonant modes of the fluid channel, and the suspended constituents are separated onto the side and center pressure nodal lines according to their sizes or acoustic contrast factors. Initial hydrodynamic focusing of the constituents within a region of the channel near to the side nodal line is a crucial step in this separation method. This new method is shown to provide a novel "parallel-stream" separation of two species of particles with good robustness. Prior numerical simulations provide essential information on this operating region and also the voltage cycle to be applied to the ultrasonic actuators for optimal separation. Experiments were conducted using a prototype of the design with polystyrene microspheres of different sizes to demonstrate the efficiency and robustness of the separation process.